Various methods are available in the prior art for protecting metallic materials surfaces against corrosive environmental influences. Coating of the metallic workpiece to be protected with a coating of another metal, for example zinc and alloys thereof, is a widespread and established method in industry. To reduce corrosion of the coating metal or prevent it for as long as possible, use is often made of conversion layers, especially on cathodically protective base coating metals such as zinc and alloys thereof. These conversion layers are reaction products of the base coating metal or an alloy thereof with the reaction solution which are insoluble in aqueous media over a wide pH range. Examples of such conversion layers are phosphating layers and chromating layers.
In the case of chromating layers, the surface to be treated is dipped into an acidic solution containing chromium(VI) ions. If the surface in question is, for example, a zinc surface, part of the zinc dissolves. Under the reducing conditions which prevail here, chromium(VI) is reduced to chromium(III) which is precipitated as, inter alia, chromium(III) hydroxide or a sparingly soluble μ-oxo- or μ-hydroxo-bridged chromium(III) complex in the, inter alia, surface film which is more alkaline as a result of the evolution of hydrogen. Sparingly soluble zinc chromate(VI) is formed in parallel. Overall, a closed, impermeable conversion layer which protects very effectively against corrosive attack by electrolytes is formed on the zinc surface.
Chromium(VI) compounds have not only an acute toxicity but a high carcinogenic potential, so that a replacement for the processes associated with these compounds is necessary.
A large number of processes using various complexes of trivalent chromium compounds have now become established as a replacement for chromating processes using hexavalent chromium compounds.
As an alternative to processes based on trivalent chromium compounds, the prior art also describes processes which make recourse to other metals for building up a conversion layer. The patent application WO 2008/119675 describes treatment solutions for producing chromium- and cobalt-free conversion layers containing oxo cations and complex halogen ions which lead to colourless and slightly iridescent layers.
However, a disadvantage of these chromium- and cobalt-free conversion layers described in the prior art is that they have hitherto existed only in colours based on interference phenomena. This encompasses virtually transparent, bluish or coloured iridescent and yellowish layers.
Dyes as are also used in Cr(III)-based passivations can in principle also be used in the case of chromium-free conversion layers.
However, here as in the case of the Cr(III)-based conversion layers, there is, owing to the low layer thickness (≦500 nm), not sufficient absorption of the light reflected by the surface over all wavelengths of visible light for the colour to be perceived as black.
In the case of black-colouring processes on the basis of Cr(III)-containing conversion layers, a transition metal such as cobalt or iron is generally added as Co(II) or Fe(II) or Fe(III) to the passivation in order to produce finely divided, black pigments in situ in the conversion layer.
Treatment solutions for producing such black passivations are described, for example, in EP 1 970 470 A1. Such treatment solutions comprise nitrate ions and at least two different carboxylic acids in addition to Cr3+ and Co2+ ions.
Iron has the disadvantage that the corrosion protection of the system is significantly weakened by incorporation of iron-containing pigments.
Although cobalt allows systems having a better corrosion protection, it has the disadvantage of being not unproblematical from a health point of view under particular conditions.
The patent text JP 2005-206872 describes a Cr6+ ion-free treatment solution for producing black passivations, which solution contains Cr3+ ions together with at least one further ion selected from the group consisting of sulphate, chloride, ions of the oxo acids of chlorine and nitrate and also a sulphur compound. The black conversion layers produced in this way contain chromium ions.
The document JP 2005-187925 describes treatment solutions for producing coloured passivation layers containing Cr3+ ions, further metal ions and an organic sulphur compound. The black conversion layers produced in this way contain chromium ions.
The document JP 2005-187838 likewise describes aqueous treatment solutions for producing black passivation layers, which solutions contain ions selected from at least Ni and Co and also a sulphur compound. The black conversion layers produced in this way contain nickel and/or cobalt ions.